Liquid toner images are developed by varying the density of pigmented solids in a developer material on a latent image bearing surface in accordance with an imaged pattern. The variations in density are produced by the corresponding pattern of electric fields extending outward from the latent image bearing surface. The fields are produced by the different latent image and background voltages on the latent image bearing surface and a voltage on a developer plate or roller.
In general, developed liquid toner images comprise carrier liquid and toner particles and are not homogeneous. Typically, a liquid toner developer contains about 1.5% to 2% solids and a developed image contains about 15% solids. The developed image has a higher density region closer to the latent image bearing surface and a "fluffy", i.e. loosely bound, region further away from the latent image bearing surface.
In order to improve transfer of a developed image from the latent image bearing surface to a substrate, it is most desirable to ensure that, before transfer, the pigmented solids adjacent background regions are substantially removed and that the density of pigmented solids in the developed image is increased, thereby compacting or rigidizing the developed image. Compacting or rigidizing of the developed image increases the image viscosity and enhances the ability of the image to maintain its integrity under the stresses encountered during image transfer It is also desirable that excess liquid be removed from the latent image bearing surface before transfer.
It is known in the prior art, as described in U.S. Pat. No. 3,955,533, to employ a reverse roller spaced about 50 microns from the latent image bearing surface to shear off the carrier liquid and pigmented solids in the region beyond the outer edge of the image and thus leave relatively clean areas above the background.
The technique of removing carrier liquid is known generally as metering. An alternative metering technique, described in U.S. Pat. Nos. 3,767,300 and 3,741,643, employs an air knife, but has not been particularly successful due to sullying of the background as a result of turbulence. Corona discharge has also been used to compress and remove liquid from a developed liquid image.
In U.S. Pat. No. 3,957,016, the use of a positive biased metering roller is proposed wherein the metering roller is maintained at a voltage intermediate the image and background voltages to clean the background while somewhat compacting the image.
In the prior art it is known to effect image transfer from a photoreceptor onto a substrate backed by a charged roller. Unless the image is rigidized before it reaches the nip of the photoreceptor and the roller, image squash and flow may occur. This is particularly true if the substrate is a non-porous material, such as plastic.
In the prior art, liquid toner images are generally transferred to substrates by electrophoresis, whereby the charged image moves from the latent image bearing surface to the substrate through the carrier liquid under the influence of an electric field produced by a high voltage, associated with the substrate, which is of opposite polarity to the charge on the image particles.
The voltage and thus the field strength available for electrophoretic transfer are limited by the danger of electrical breakdown which can occur at both the input and output edges of the nip, due to the minimum of the Paschen curve being at about 8 microns. Thus, according to the Paschen curve, the voltage difference at the nip preferably should not exceed about 360 volts, in order to avoid electrical breakdown and possible damage to the image and latent image bearing surface.
Electrophoretic compaction of images prior to transfer thereof is described in U.S. Pat. No. 4,286,039 which shows a metering roller followed by a negatively biased squeegee roller. The squeegee roller is operative both for compacting the image and for removing excess liquid.
U.S. Pat. Nos. 4,690,539 and 4,708,460 describe apparatus for removing substantially all of the carrier liquid from a liquid image on an image transfer member, prior to transfer to the final substrate.
U.S. Pat. No. 4,684,238 describes the use of an electrified roller spaced away from a liquid image on an intermediate transfer member. The stated object of this mechanism is the compacting of the image and the removal of liquid therefrom.
U.S. Pat. No. 4,796,048 describes a system for transferring a liquid toner image from a photoconductor to an image transfer member. The image transfer member is urged against the photoconductor during transfer to squeegee carrier liquid away from the non-image areas. The image areas are kept in a spaced relationship from the intermediate transfer member by spacer particles in the toner material as described in U.S. Pat. No. 4,582,774. This toner material is the only toner described in U.S. Pat. No. 4,796,048 as being a suitable toner.